1. Field of the Invention
The present invention relates to an electrode used in lithium-ion secondary batteries and the like and to a method for producing the electrode.
2. Description of Related Art
Nonaqueous electrolyte batteries typified by a lithium-ion secondary battery are characterized by their high energy density, and thus have been widely used as power sources for portable devices such as a portable telephone and a notebook personal computer. There is a trend toward a further increase in the capacity of lithium-ion secondary batteries as the portable devices have become more sophisticated. Hence, it is necessary to further increase their energy density.
As one of the means of increasing the energy density of a lithium-ion secondary battery, an active material-containing layer formed on one side or both sides of a current collector has been increased in thickness. However, an increase in the thickness of the active material-containing layer leads to an increase in the distance from the active material-containing layer to the current collector, which makes it difficult for an electrolyte to permeate into the vicinity of the current collector from the surface of the active material-containing layer, so that the charge-discharge characteristics may not be improved.
To solve this problem, JP 2008-251401 A, for example, proposes an electrode in which the active material particle size distribution in the lower portion of the active material-containing layer (i.e., the portion of the active material-containing layer close to the current collector) has a larger peak than that of the active material particle size distribution in the surface portion of the active material-containing layer (i.e., the portion of the active material-containing layer on the side opposite to the current collector) and the thickness of the lower portion makes up 50 to 90% of the total thickness of the surface portion and the lower portion.
In the electrode described in JP 2008-251401 A, while the electrolyte permeability improves in the surface portion of the active material-containing layer because the porosity of the surface portion increases, the electrolyte permeability tends to deteriorate in the lower portion of the active material-containing layer because the porosity of the lower portion declines contrary to the surface portion.
With the foregoing in mind, the present invention provides an electrode in which the active material-containing layer has improved electrolyte permeability in the lower portion.